A variety of methods have been employed to utilize the compression of an internal-combustion engine for braking when a vehicle is moving down a grade. One such method is disclosed by Custer in U.S. Pat. No. 4,398,510. In the device disclosed in the Custer patent, the exhaust valves of an internal-combustion engine are opened at a point in the compression stroke when they would normally remain closed.
To ensure proper operation of an internal-combustion engine, a minimum cold clearance, typically on the order of 0.018 inch, is maintained in the valve-actuating mechanism of the engine. This clearance is necessary to prevent premature opening of the exhaust valves when the engine becomes hot. The improvement disclosed by Custer is an anti-lash timing mechanism which takes up this cold clearance during brake operation to improve the opening and closing of the exhaust valves during the various cycles of the engine. The timing mechanism of Custer displaces a slave piston to take up the cold clearance when the brake is in operation so that a high-pressure pulse of engine oil from a master piston driven off a fuel injector camshaft can open the exhaust valves at the appropriate time.
The Custer timing mechanism, however, is too complex. The mechanism employs two coaxial springs, a ball check valve within an inner closely fitting piston, and various pins to limit the degree of lash take-up. This structure is necessary to axially extend and lock the piston in its extended position. Thus, a need exists for a different compression relief brake operating mechanism which can control the operation of the exhaust valves during brake operation with a minimum of moving parts and close tolerances.
My previous U.S. Pat. No. 4,655,178 issued Apr. 7, 1987, discloses an Anti-Lash Adjuster. However, it has been found that in some cases at least, such as with Cummins engines, it is difficult to fit a compression spring of the required strength in the available space. This could be compensated for by reducing the strength of the return spring, but this could cause engine damage because the slave piston could be jacked down by engine oil pressure which overcomes the weaker return spring. On each cycle the exhaust valves are fully opened on the exhaust stroke. With the force of the valve springs thus removed, the oil pressure may be enough to move the slave piston down, while return is prevented by the check valve. Thus the slave piston may be jacked down over a period of time until the exhaust valves are opened enough to strike the pistons of the engine.